Professional Java Server Programming J2EE Edition
Chapter 12
This amount of access equals power. Note, though, that it is poor style to modify request and response
directly from a tag handler. Custom tags should be thought of as generic building blocks intended for use in a
wide variety of contexts. In practice tags should not be concerned with the parameters passed to the JSP page.
Although a tag handler can access request parameters, relying on doing so will greatly reduce its reusability.
The Simple Example Revisited
To catch our breath after all this theory, let's look again at the Java implementation of the simple example we
introduced earlier. We see that the tag handler extends TagSupport, and so gets most of its functionality for
free. It has no state and accesses no file or other resources, so there is no need to override release(). We
use doEndTag() to access the PageContext, obtain a JspWriter, and generate output:
package tagext;
import java.io.IOException;
import java.util.Date;
import javax.servlet.jsp.*;
import javax.servlet.jsp.tagext.*;
// Implementation of a tag to generate a single piece of HTML.
public class HelloTag extends TagSupport {
// This method will be called when the JSP engine encounters the start
// of a tag implemented by this class
public int doStartTag() throws JspTagException {
// This return value means that the JSP engine
// should evaluate the contents and any child tags of this tag
return EVAL_BODY_INCLUDE;
}
// This method will be called when the JSP engine encounters the end
// of a tag implemented by this class
public int doEndTag() throws JspTagException {
String dateString = new Date().toString();
try {
pageContext.getOut().write("Hello world.<br/>");
pageContext.getOut().write("My name is " + getClass().getName() +
" and it's " + dateString + "<p/>");
}
catch (IOException ex) {
throw new JspTagException
("Fatal error: hello tag could not write to JSP out");
}
// This return value means that the JSP engine
// should continue to evaluate the rest of this page
return EVAL_PAGE;
}
} // class HelloTag
Note that we need to check for IOExceptions when generating output. Any exception encountered while
processing the tag must be wrapped as a JspException if it is to be rethrown; it is good practice to use the
javax.servlet.jsp.JspTagException subclass of JspException. (Note that, confusingly, this isn't
in the same package as the other classes specific to tag handlers.)
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